Presently, digital communications systems include systems for communicating audio information, video information and other data. These systems include methods for distributing the information over coaxial cable, wireless connections, etc. Some such digital communication systems use digital modulation methods to transmit digital bit-streams in a limited frequency band. The digital information, typically in a form of bit-streams, are initially modulated onto a digital baseband signal, then converted to an analog baseband signal. The analog baseband signal is then converted to a radio frequency (RF) signal. Alternatively, digital information may first be converted to analog information (i.e., an analog baseband signal). The analog baseband signal is then modulated onto an RF carrier. This is typically accomplished by the means of a well known quadrature I, Q modulator. FIG. 1 shows a block diagram of a prior art digital-to-analog converter (DAC) 100 for performing this function.
The DAC system 100 in FIG. 1 receives an in-phase digital signal, I. The system 100 also receives a quadrature-phase digital signal, Q. The digital signals, I and Q take the form of a stream of digital values (or samples) that represent the amplitude of the baseband I and Q signal, respectively, at a series of points in time. The digital sample streams I, Q are converted by DACs 102, 104 to a step-wise function that approximates the analog representation of the baseband signals, I and Q formerly represented by the digital sample streams. The DACs 102, 104 receive a sample clock signal 106 having a sampling frequency, fs. The sample clock signal 106 is generated by a clock generator 108 and an oscillator 110. Ideally, the DACs 102, 104 would exactly recreate the analog signals I and Q. However, the conversion process in the DACs 102, 104 introduces distortion. The distortion is in the form of sin(x)/x (commonly known as the “sinc” function). The input baseband signal is multiplied by the sinc function in the frequency domain. As such, the DAC output signals 112, 114 have a sinc envelope.
In addition to the multiplication by the sinc function, the output of the DACs 102, 104 include the spectral images having a frequency equal to the sampling frequency, fs plus and minus the frequency of the baseband signals I and Q. In addition, spectral images are generated at each harmonic of the sampling frequency fs plus and minus the baseband frequency. The output 112, 114 of the DACs 102, 104 are upconverted by the means of multiplication in upconverters or mixers 124, 126, typically using quadrature local oscillator (LO) signals 127, 128. The process of multiplication accomplishes both the modulation of the analog baseband signal onto the RF carrier, and upconversion of the baseband signal to the RF carrier frequency. The first local oscillator (LO) signal 127 is generated by an oscillator 130. The second LO signal 128 is generated by a minus 90 degree phase shifter 132. The oscillator 130 is typically independent of the oscillator 110 used to generate the sample clock signal 106 in order to make a greater frequency range of carrier frequencies available. After upconversion, an RF output signal 136 is generated by subtracting the mixer output signal 142 from the mixer output signal 140 using a subtraction device 138.
In addition to fundamental frequency, the LO signals 127 and 128 typically contain harmonic frequencies (or the LO harmonics may be generated inside the mixers in the multiplication process). Often, the LO signal is a square-wave, with strong harmonic content, predominantly of odd-order. If raw, unfiltered DACs output signals 112 and 114 were directly applied to their respective mixers, the spectral images generated by the DACs 102, 104 may be converted by the LO fundamental and its harmonics to the desired frequency and thus interfere with the desired upconverted output from the mixers 124, 126. Furthermore, the prior art DAC system 100 would not be able to generate a single-sideband (SSB) signal, because the cancellation of the other sideband in the subtracting junction 138 would be incomplete due to unwanted converted terms falling on the that sideband, i.e. a residual power would remain in the other (unwanted) sideband. It is well known in the art that the ability to generate SSB signal is essential in order to be able to construct complex RF signals.
In order to prevent spectral contamination and achieve the ability to generate SSB signals (thus any complex signal), low pass anti-aliasing or reconstruction filters (LPRF) 116, 118 are used to reduce or eliminate the undesired spectral images from the DAC output signals 112, 114. However, LPRFs are large and complex, driving up the cost of such DAC systems. In particular, the requisite active filters in LPRFs increase the cost, power and die size of a chip. LPRF filters also present performance difficulties. For example, it is desirable to carefully balance and match the I-channel and Q-channel signal paths. This is made more difficult by the use of LPRFs. Particularly, higher order LPRF filters make balancing and matching the signal paths more difficult. The large LPRFs also compete with design-critical blocks for optimal placement on the die. All of these factors present design challenges that may significantly impact cost and the performance of DAC systems.
It can be seen from the above description that there is a need for techniques to directly generate bandpass signals while avoiding the aforementioned issues of conventional approaches.